The present invention relates generally to an improved method of medical imaging over large areas, and more particularly, to a method and apparatus of acquiring magnetic resonance (MR) images over an area that is greater than the optimal imaging area of an MR scanner using step-wise movement of a table through the MR scanner without incurring slab-boundary artifacts.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, MZ, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In such MRI systems, the volume for acquiring MR data with optimal gradient linearity, having a uniform magnetic field B0, and uniform radio frequency (RF) homogeneity is of limited extent. Desired fields-of-view (FOV) that exceed this limited volume are traditionally acquired in sections, with table motion between scans. The resulting concatenated images often exhibit discontinuities at the slab junctions. These slab-boundary artifacts result in non-ideal images. When these artifacts are either severe or occur in a critical region-of-interest, complete re-acquisition of data may be needed for a thorough analysis.
It would therefore be desirable to have a new method and apparatus that allows coverage of large FOV without slab-boundary artifacts in the resulting concatenated images.
The present invention relates to a system and method of acquiring large FOV MR images using incremental table motion for increased volume coverage that results in reconstructed images without discontinuities.
Slab-boundary artifacts are eliminated in a 3D acquisition sequence by stepping an imaging object with respect to the optimal volume of the imaging apparatus, or vice versa. For example, stepping a moveable table in small increments and using a unique acquisition and reconstruction scheme. The thickness of the slab, which is smaller than the desired FOV, is selected to remain within the optimal volume of the MR system. The selected slab position remains fixed relative to the magnet of the MR scanner during the scan, and the table is moved in incremental steps during scanning of the entire FOV. MR data is acquired by applying an excitation that excites spins and applying magnetic field gradient waveforms to encode the volume of interest. The volume of interest is restricted in the direction of table motion. The magnetic field gradients traverse k-space following a trajectory that is uniform in the k-space dimension that is in the direction of table motion. The magnetic field gradient waveforms that encode the k-space directions perpendicular to table motion are divided into subsets. At each table position, all the k-space data in the direction of table motion are acquired for a subset of the remaining two k-space dimensions. After acquisition, data is transformed in the direction of table motion, sorted, and aligned to match anatomical locations in the direction of table motion. This procedure is repeated to fill the entire 3D matrix. A final image is reconstructed by transforming the data in the dimensions perpendicular to table motion. This stepped table approach provides reconstructed images absent of slab-boundary artifacts over large FOV imaging areas.
In accordance with one aspect of the invention, a method of imaging large volumes without resulting slab-boundary artifacts includes defining a desired FOV larger than an optimal imaging volume of an MR scanner and selecting a slab thickness in a first direction that is smaller than the desired FOV but that is within the optimal imaging volume of the MR scanner. MR data is then acquired by exciting and encoding spins to acquire data that is restricted to the selected slab thickness. The imaging area is then moved step-wise with respect to the imaging object using a step distance that is preferably a multiple of the image resolution in the direction of table motion. This process is repeated until the desired FOV is fully encoded using a series of cyclically repeated magnetic field gradient waveforms. Scan time can be minimized by applying time-varying magnetic field gradient waveforms during signal acquisition. The selected trajectory need only be uniform in the direction of motion. Artifacts are reduced by using stepped acquisitions as opposed to continuous motion acquisitions.
In accordance with another aspect of the invention, an MRI apparatus is disclosed to acquire multiple sets of MR data with a moving table and reconstruct MR images without slab-boundary artifacts that includes a magnetic resonance imaging system having an RF transceiver system and a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field. An RF switch is controlled by a pulse module to transmit and receive RF signals to and from an RF coil assembly to acquire MR data. A patient table is moveable fore and aft in the MR system about the magnetic bore to translate the patient so that an FOV larger than the optimal scanning area of the MRI system can be scanned. A computer is programmed to receive input defining a desired FOV larger than the optimal imaging volume of the MR system. The computer is also capable of defining a fixed slab with respect to the magnet to acquire the MR data therein. The computer then acquires full MR data in the direction of table motion for a subset of the data in the directions perpendicular to table motion, and then increments position of the patient table while maintaining the position of the fixed slab. The algorithm is repeated, collecting the necessary MR data across the defined FOV. To reconstruct the image, the MR data is first transformed in the direction of table motion, and then aligned to match anatomic locations across slab boundaries. Thereafter, the MR data is transformed with respect to the remaining two dimensions to reconstruct an MR image.
Yet another aspect of the invention includes a computer program having a set of instructions executed by a computer to control a medical imaging scanner and create images across scanning boundaries without significant boundary artifacts. The computer is caused to select an FOV spanning an area greater than a predefined optimal image area of the medical imaging scanner, apply an RF pulse to excite a region of interest, and apply magnetic field gradients to encode the region of interest in a first direction. The volume of interest is limited in the direction of table motion either by using a slab-selective RF pulse or by acquiring the data in such a way that an acquisition filter can be used to restrict the spatial extent. Three-dimensional image data can then be acquired in the first direction as a subset of a second and third direction for each table movement. The computer then causes a patient table to move an incremental step with respect to the medical imaging scanner, and repeat the image data acquisition in the first direction for he remaining two directions until sufficient data is acquired across the entire FOV. An image can be reconstructed without slab-boundary artifacts after aligning anatomical data in the first direction.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.